Thrusters for propulsion units which are designated arc jets by the specialized community, have become known through the publication "Cathode Erosion Tests for 30 kW Arcjets" by W.D. Deininger, A. Chopra and K.D. Goodfellow, A/AA 89-2264, among other publications, which was published on the occasion of the 25th Joint Propulsion Conference A/AA/ASME/SAE/ASEE, Monterey, CA, July 10-12, 1989. The propulsion gas, which typically may be ammonia NH.sub.3 or a mixture of ammonia, nitrogen N.sub.2 and hydrogen gas H.sub.2, which is produced by a thermal and/or catalytic decomposition of hydrazine N.sub.2 H.sub.4, is directed in these prior art thrusters through a bore which is disposed in the side wall of a plenum chamber and into the plenum chamber itself. The gas, which has a temperature on the order of 500.degree.-600.degree. C., upon its entry into the plenum chamber, is heated by an electric arc, which is formed between the anode (positive terminal) and the cathode's (negative terminal), to temperatures of 10.degree.-15,000.degree. C. prior to leaving the expansion nozzle and producing the desired forward thrust or propulsion effect.
The electric arc, which builds up after ignition in the gas flow of the thruster, extends from the anode (positive terminal) to the generally cone-shaped tip portion of the cathode (the negative terminal) through the neck of the nozzle or the constricted region. The electric arc, which exists between the anode and the cathode, transfers the largest portion of its kinetic energy to the rotating propulsion gas in the nozzle's constricted region. The electric arc is formed precisely along the central axis of the constricted region.
The region, in which the electric arc emerges from the inside wall of the expansion nozzle, is highly stressed thermally by wall temperatures which exceed 2000.degree. C. In order to achieve as stable an electric arc as possible, it is desired to provide a rotating flow of the propulsion gases in the nozzle neck or constricted region. A high rotational velocity for the propulsion gas is also an important precondition for a tight gas flow which is produced in the region of the constricted region of the nozzle neck, and because of the high velocity during rotation, for as much heat as possible being transferred from the electric arc to the gas.
In order to obtain a rotating gas stream, the propulsion gas in such thrusters is introduced into the plenum chamber with a tangential flow component, through bores arranged in the side wall of the plenum chamber or in an annularly-shaped insulator, which is provided between the cathode and the anode surrounding the cathode.